The other day I was using a full licensed version Adobe Acrobat X as part of the CS6 suite and it suddenly stopped working. The only solution that I found was to reinstall which was a pain. It happened again and I found another solution recently. The following solution is a lot easier to use but still a pain.

1. Make sure you are connected to the internet.
2. Launch a regular CS6 product other than Acrobat or Flash Builder (for example: Photoshop, After Effects, Premier Pro, InDesign, Flash Professional, Illustrator, Dreamweaver.)
3. Click on Help->Deactivate .
4. Deactivate the application. Quit application.
5. Launch a regular CS6 product other than Acrobat or Flash Builder (for example: Photoshop, After Effects, Premier Pro, InDesign, Flash Professional, Illustrator, Dreamweaver.)
   Accept Eula.
6. Register trial by signing in. Let the app launch.
7. Quit and relaunch the app. Wait for UI to come.
8. Click on License This Software button on the UI.
9. Click on Sign in .
10. Serial number screen will come
11. Enter the serial number in the serial number screen and click Next.
12. Quit app and relaunch the app while connected on the internet .

Source: http://forums.adobe.com/message/4506235